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Date:   Tue, 5 Jul 2022 10:21:16 -0700
From:   Luiz Augusto von Dentz <luiz.dentz@...il.com>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>, stable@...nel.org,
        Marcel Holtmann <marcel@...tmann.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "linux-bluetooth@...r.kernel.org" <linux-bluetooth@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [RESEND 1/1] Bluetooth: Use chan_list_lock to protect the whole
 put/destroy invokation

Hi Lee,

On Wed, Jun 29, 2022 at 8:28 AM Lee Jones <lee.jones@...aro.org> wrote:
>
> On Tue, 28 Jun 2022, Luiz Augusto von Dentz wrote:
>
> > Hi Eric, Lee,
> >
> > On Mon, Jun 27, 2022 at 4:39 PM Luiz Augusto von Dentz
> > <luiz.dentz@...il.com> wrote:
> > >
> > > Hi Eric, Lee,
> > >
> > > On Mon, Jun 27, 2022 at 7:41 AM Eric Dumazet <edumazet@...gle.com> wrote:
> > > >
> > > > On Wed, Jun 22, 2022 at 10:27 AM Lee Jones <lee.jones@...aro.org> wrote:
> > > > >
> > > > > This change prevents a use-after-free caused by one of the worker
> > > > > threads starting up (see below) *after* the final channel reference
> > > > > has been put() during sock_close() but *before* the references to the
> > > > > channel have been destroyed.
> > > > >
> > > > >   refcount_t: increment on 0; use-after-free.
> > > > >   BUG: KASAN: use-after-free in refcount_dec_and_test+0x20/0xd0
> > > > >   Read of size 4 at addr ffffffc114f5bf18 by task kworker/u17:14/705
> > > > >
> > > > >   CPU: 4 PID: 705 Comm: kworker/u17:14 Tainted: G S      W       4.14.234-00003-g1fb6d0bd49a4-dirty #28
> > > > >   Hardware name: Qualcomm Technologies, Inc. SM8150 V2 PM8150 Google Inc. MSM sm8150 Flame DVT (DT)
> > > > >   Workqueue: hci0 hci_rx_work
> > > > >   Call trace:
> > > > >    dump_backtrace+0x0/0x378
> > > > >    show_stack+0x20/0x2c
> > > > >    dump_stack+0x124/0x148
> > > > >    print_address_description+0x80/0x2e8
> > > > >    __kasan_report+0x168/0x188
> > > > >    kasan_report+0x10/0x18
> > > > >    __asan_load4+0x84/0x8c
> > > > >    refcount_dec_and_test+0x20/0xd0
> > > > >    l2cap_chan_put+0x48/0x12c
> > > > >    l2cap_recv_frame+0x4770/0x6550
> > > > >    l2cap_recv_acldata+0x44c/0x7a4
> > > > >    hci_acldata_packet+0x100/0x188
> > > > >    hci_rx_work+0x178/0x23c
> > > > >    process_one_work+0x35c/0x95c
> > > > >    worker_thread+0x4cc/0x960
> > > > >    kthread+0x1a8/0x1c4
> > > > >    ret_from_fork+0x10/0x18
> > > > >
> > > > > Cc: stable@...nel.org
> > > >
> > > > When was the bug added ? (Fixes: tag please)
> > > >
> > > > > Cc: Marcel Holtmann <marcel@...tmann.org>
> > > > > Cc: Johan Hedberg <johan.hedberg@...il.com>
> > > > > Cc: Luiz Augusto von Dentz <luiz.dentz@...il.com>
> > > > > Cc: "David S. Miller" <davem@...emloft.net>
> > > > > Cc: Eric Dumazet <edumazet@...gle.com>
> > > > > Cc: Jakub Kicinski <kuba@...nel.org>
> > > > > Cc: Paolo Abeni <pabeni@...hat.com>
> > > > > Cc: linux-bluetooth@...r.kernel.org
> > > > > Cc: netdev@...r.kernel.org
> > > > > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > > > > ---
> > > > >  net/bluetooth/l2cap_core.c | 4 ++--
> > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> > > > > index ae78490ecd3d4..82279c5919fd8 100644
> > > > > --- a/net/bluetooth/l2cap_core.c
> > > > > +++ b/net/bluetooth/l2cap_core.c
> > > > > @@ -483,9 +483,7 @@ static void l2cap_chan_destroy(struct kref *kref)
> > > > >
> > > > >         BT_DBG("chan %p", chan);
> > > > >
> > > > > -       write_lock(&chan_list_lock);
> > > > >         list_del(&chan->global_l);
> > > > > -       write_unlock(&chan_list_lock);
> > > > >
> > > > >         kfree(chan);
> > > > >  }
> > > > > @@ -501,7 +499,9 @@ void l2cap_chan_put(struct l2cap_chan *c)
> > > > >  {
> > > > >         BT_DBG("chan %p orig refcnt %u", c, kref_read(&c->kref));
> > > > >
> > > > > +       write_lock(&chan_list_lock);
> > > > >         kref_put(&c->kref, l2cap_chan_destroy);
> > > > > +       write_unlock(&chan_list_lock);
> > > > >  }
> > > > >  EXPORT_SYMBOL_GPL(l2cap_chan_put);
> > > > >
> > > > >
> > > >
> > > > I do not think this patch is correct.
> > > >
> > > > a kref does not need to be protected by a write lock.
> > > >
> > > > This might shuffle things enough to work around a particular repro you have.
> > > >
> > > > If the patch was correct why not protect kref_get() sides ?
> > > >
> > > > Before the &hdev->rx_work is scheduled (queue_work(hdev->workqueue,
> > > > &hdev->rx_work),
> > > > a reference must be taken.
> > > >
> > > > Then this reference must be released at the end of hci_rx_work() or
> > > > when hdev->workqueue
> > > > is canceled.
> > > >
> > > > This refcount is not needed _if_ the workqueue is properly canceled at
> > > > device dismantle,
> > > > in a synchronous way.
> > > >
> > > > I do not see this hdev->rx_work being canceled, maybe this is the real issue.
> > > >
> > > > There is a call to drain_workqueue() but this is not enough I think,
> > > > because hci_recv_frame()
> > > > can re-arm
> > > >    queue_work(hdev->workqueue, &hdev->rx_work);
> > >
> > > I suspect this likely a refcount problem, we do l2cap_get_chan_by_scid:
> > >
> > > /* Find channel with given SCID.
> > >  * Returns locked channel. */
> > > static struct l2cap_chan *l2cap_get_chan_by_scid(struct l2cap_conn
> > > *conn, u16 cid)
> > >
> > > So we return a locked channel but that doesn't prevent another thread
> > > to call l2cap_chan_put which doesn't care about l2cap_chan_lock so
> > > perhaps we actually need to host a reference while we have the lock,
> > > at least we do something like that on l2cap_sock.c:
> > >
> > > l2cap_chan_hold(chan);
> > > l2cap_chan_lock(chan);
> > >
> > > __clear_chan_timer(chan);
> > > l2cap_chan_close(chan, ECONNRESET);
> > > l2cap_sock_kill(sk);
> > >
> > > l2cap_chan_unlock(chan);
> > > l2cap_chan_put(chan);
> >
> > Perhaps something like this:
>
> I'm struggling to apply this for test:
>
>   "error: corrupt patch at line 6"

Check with the attached patch.


-- 
Luiz Augusto von Dentz

View attachment "0001-Bluetooth-L2CAP-WIP.patch" of type "text/x-patch" (3622 bytes)

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